Photolithography is one of the important steps in the semiconductor manufacturing technology, which utilizes an exposure process and a developing process to form patterns in a photoresist layer. With the continuous increase of the integration level of IC chips, the critical dimension of features to be formed by a photolithography process has become smaller and smaller.
The minimum critical dimension of the features formed by a photolithography process may be determined by the resolution (R) of an exposure apparatus. The resolution of the exposure apparatus is described by an equation: R=Kλ/(NA), wherein “K” refers to a coefficient related to an exposure process; “λ” refers to the wavelength of the exposure light; and “NA” refers to the numerical aperture of the optical system of the exposure apparatus. According to such an equation, there may be two approaches to increase the resolution of the exposure apparatus: increasing the numerical aperture of the optical system; and reducing the wavelength of the exposure light.
A lot of efforts have been made on increasing the numerical aperture of the optical system of the exposure apparatus. However, because the next generation photolithography technique has strict requirement for the minimum critical dimension, it often requires the optical system to have a large numerical aperture. Thus, it may cause the manufacturing and modulating of the optical system to be complex, and further increasing the numerical aperture may have a significant limitation on the focus depth of the optical system.
Therefore, the other approach, reducing the wavelength of the exposure light, has been deployed to increase the resolution of the exposure system. Extreme ultraviolet (EUV) source is a recently developed light source. The wavelength of the EUV light is approximately 13.5 nm, or shorter. When the EUV light source is used in an exposure system, the critical dimension of the formed features may be substantially small. The mainstream method for generating EUV light is to use Laser Produced Plasma (LPP). The principle of LPP is to bombard a target made of Sn to generate plasma; and the plasma radiates ultraviolet light.
FIG. 1 illustrates an existing EUV light source. As shown in FIG. 1, the EUV light source includes a Sn spray nozzle 101. The Sn spray nozzle 101 sprays Sn droplets 102 downwardly. The EUV light source also includes a laser source 103. The laser source 103 generates a laser beam 104. The laser beam 104 is focused by the lens unit 105; and is used to bombard the Sn droplets 102. The bombarded Sn droplets 102 generates plasma; and the plasma radiates EUV light 108. Further, the EUV light source also includes a single-piece focusing mirror 107. The focusing mirror 107 collects the radiated EUV light 108; and further focuses the collected EUV light 108 at a central focusing point 109.
However, the power of the EUV light generated by such EUV light source is relative low, and it may be unable to match the manufacturing requirements. The disclosed EUV light source and exposure apparatus and other aspects of the present disclosure are directed to solve one or more problems set forth above and other problems.